Not Applicable
Not Applicable
This invention relates to color video projection displays and more particularly to employing multiple color wheels and a segmented light pipe to increase the luminous efficiency of single light valve color video projection displays.
1,200 lumens is an accepted brightness goal for color video projectors. A contrary goal is to employ the lowest projection lamp power possible to reduce cooling fan noise and power supply size and cost. Both goals can be achieved by substantially increasing the lumens per watt efficiency (hereafter xe2x80x9cluminous efficiencyxe2x80x9d) of the projection display.
A commonly employed way of improving the luminous efficiency of color video projectors is by optically dividing the projection lamp illumination into three separate pathways, one for each primary color, providing a light valve in each pathway, modulating each light valve with its respective color data, and recombining the three pathways into a converged projected color image. Such three-path color video projectors have suitable luminous efficiency, but are generally costly, large, heavy, optically complex, and require precision alignment.
Prior workers have addressed the above-described problems by employing frame sequential illumination through a single light path that color timeshares a single light valve. Current single light valve projectors are relatively inexpensive, light weight, and compact. Such projectors provide color sequential illumination of the light valve by typically employing a color wheel which, unfortunately, transmits only about 30 percent of the projection lamp illumination at any time. Such projectors typically employ about a 120 watt projection lamp, which results in a brightness of only about 600-800 lumens.
U.S. Pat. No. 5,548,347 for SINGLE PANEL COLOR PROJECTION VIDEO DISPLAY HAVING IMPROVED SCANNING, and U.S. Pat. No. 5,845,981 for MULTI-COLOR-BAND SCROLLING ACROSS SINGLE-PANEL LIGHT VALVE describe systems for increasing the luminous efficiency of single path color video projectors by dividing the projection lamp illumination into three primary colors and passing the primary colors through rotating prisms to scroll the resulting color bands across a single light valve. Color data driving the light valve is scrolled in synchronism with the prism rotation to project a color image. Unfortunately, such projectors are are relatively costly, heavy, and optically complex. Moreover, color purity depends on scrolling the color bands across the light valve in precise synchronism with the prism rotation. Accordingly, color purity is ensured by optically separating the scrolling color bands with dark xe2x80x9cguard bandsxe2x80x9d which, unfortunately, reduces the luminous efficiency.
An object of this invention is, therefore, to provide an apparatus and a method for increasing the luminous efficiency of a single-path color video projector.
Another object of this invention is to provide a segmented light pipe for enabling three-color area-division multiplexing of a single light valve.
A further object of this invention is to provide multiple color wheel embodiments for illuminating the light pipe segments with alternating primary colors.
Still another object of this invention is to provide a color video projector that projects a 1,200 lumen image with a 120 watt projection lamp.
A color sequential video projector of this invention employs a color modulating device and a segmented light pipe that coact to at least double the lumens per watt efficiency of the projector relative to projectors employing conventional color wheels. This invention employs a color modulating device that splits polychromatic light into three different colored light beams that are positionally stationary, but alternate mutually exclusive colors sequentially with time such that each pixel of a light valve is exposed to all three colors over an image frame time. A segmented light pipe receives the three light beams and forms three adjacent color bands that are precisely aligned on the light valve. Because all three colors of light are constantly illuminating the light valve, albeit in a rapidly alternating manner, the approximately 60 percent color wheel light attenuation is substantially eliminated.
The color modulating device employs multiple color wheels each having filter segments that form the three light beams by reflecting the alternating, mutual exclusive colors as the wheels rotate. Alternative embodiments employ one, two, or three color wheels having flat or conically-shaped filter segment surfaces.
The segmented light pipe includes three rectangular glass slabs surrounded by a low index cladding and having their abutting surfaces coated with a very thin, low refraction index coating. The light pipe input apertures receive three roughly shaped light beams, homogenize them, and provide at three output apertures uniformly bright rectangular light beams. A precision achromatic lens images the uniform light beams onto the light valve such that seams between the beams are imaged to within one micron of accuracy and are parallel to the light valve pixel rows.
Additional objects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof that proceed with reference to the accompanying drawings.